PROJECT SUMMARY/ABSTRACT The ability of neurons to selectively synapse with correct cell types amidst many alternatives (here referred to as synaptic specificity) underlies the structure and function of the nervous system. With respect to progress made in illuminating mechanisms governing the guidance and patterning of axons and dendrites, our knowledge of how synaptic specificity is achieved is severely limited. Addressing this gap in knowledge is essential to understanding how the precision of neural connectivity is established. Our goal is to identify general molecular strategies underlying synaptic specificity. Progress in this area has been limited by the difficulty in studying synapse formation with precise molecular and cellular resolution in complex regions. Therefore, we focus on the Drosophila visual system, wherein cell types and synapses between them are well- characterized, and it is feasible to interrogate gene function in a cell autonomous manner. It is widely believed that neurons identify correct synaptic partners through use of complementary cell surface tags that function like a ?lock and key?. However, evidence supporting this idea is scarce. Previously, we found that members of two subfamilies of the immunoglobulin superfamily (IgSF), dprs (21 members) and dpr-interacting proteins (DIPs) (9 members) which bind heterophilically, are expressed in a matching manner between synaptic partners in the Drosophila visual system. Based on our preliminary findings, we hypothesize that dpr-DIP interactions regulate synaptic specificity by biasing synapse formation towards specific cell types, thereby preventing promiscuous synapse formation with incorrect partners. In this model, dpr-DIP interactions are not necessary for synaptogenesis, but promote synapse formation between specific cell types, potentially by controlling the location of synaptic machinery. We will test this hypothesis in 3 Specific AIMs. In AIMs I and II, we perform focused studies at specific synapses in the lamina to determine if dpr-DIP interactions (I) are necessary to prevent synapse formation with incorrect partners, and (II) have the capacity to promote synapse formation between specific cell types. In AIM III, we will test whether dpr-DIP interactions generally control synaptic connectivity in the visual system through broader studies in a different region of the optic lobe (medulla), which address (1) the function of diverse dpr-DIP interactions at multiple synapses, and (2) whether complementary dpr/DIP expression is generally predictive of synaptic connectivity. In general, our data support the longstanding idea that neurons identify correct synaptic partners through complementary cell surface tags that function like a ?lock and key?. However, we propose that such molecules are not necessary for synaptogenesis, and rather control synaptic specificity by limiting promiscuous synapse formation. This research will advance fundamental knowledge of how neurons selectively form synapses. As dpr-DIP complexes are similar to complexes of mammalian IgSF proteins our findings will be widely transferable. In the long-term, we expect our findings to inform strategies for restoring brain function in the context of disease.